Diffusion coefficient and electromigration velocity of copper in thin silver films

Abstract

The objective of this work was to determine the Cu diffusion coefficient in silver thin films as a function of temperature, and Cu electromigration velocity as a function of electric current density and to test whether linearity between electromigration and current density was preserved at high current stress. An Ag stripe was doped with Cu in a limited region, and the movement of Cu out of this region under temperature and current stressing was measured by electron probe. The experiment was conducted at 200, 250, and 300°C and at current densities of 106, 2×106, and 3×106 A/cm2. From the copper concentration profile, the diffusion coefficient D and the electromigration velocity V were calculated. Results show that the diffusion of Cu in Ag films obtained by extrapolation to use conditions (D=10−15 cm2/sec at 55°C) is about 2 orders of magnitude greater than bulk, and that electromigration under ∼106−A/cm2 stress is negligible when compared to thermal diffusion of Cu, even at ambient temperature. At high current density (3×106 A/cm2), the electromigration velocity becomes much greater than the value predicted on the basis of linearity between V and J. This indicates a departure from the Nernst-Einstein relationship of ion shift in a field.